Mechanistic Insights into the In Situ Restructuring of Coordinated Copper in Postmetalated MOFs for Photocatalysis

Metal-organic frameworks (MOFs) offer a powerful platform for the rational design of photocatalysts, where systematic structural tuning can be employed for efficient solar-to-chemical energy conversion. Here, we establish a direct structure-activity relationship in a series of copper-metalated mixed-linker UiO-66 derivatives, UiO-66(COOH)_x-Cu (0 ≤ x ≤ 2), incorporating increasing densities of free carboxylate groups in their MOF backbone. These functionalities determine both Cu coordination and in situ restructuring under the photocatalytic dehydrogenation of formic acid (FAc). Operando Fourier-transform infrared (FTIR) and X-ray absorption spectroscopy (XAS) reveal that the -COOH content dictates the evolution of the surface of the framework and Cu speciation during photocatalysis. FTIR demonstrates that intraframework anhydride formation correlates linearly with photocatalytic efficiency, while XAS evidences a light-induced restructuring of coordinated Cu(II)/Cu(I) species into a catalytically active Cu(I)/Cu(0) binary system in the presence of FAc. Time-resolved spectroscopy further indicates a photoinduced charge transfer from the Zr-oxo clusters to the Cu centers, enhancing the overall reactivity. Density functional theory (DFT) calculations corroborate these findings, showing that the number and spatial arrangement of carboxylates influence the Cu coordination stability and its restructuring dynamics. These insights reveal how linker functionalization governs metal speciation and dynamics in MOFs, offering design principles for next-generation light-driven catalytic systems.